Pitch command signals have been formulated heretofore as shown in U.S. Pat. No. 3,822,047 issued July 2, 1974. In contrast, a preferred embodiment of the present invention provides utilization of aircraft pitch angle data in an automatic braking system.
Heretofore, utilization of the autobrake system on aircraft has tended to derotate the aircraft following touchdown at a faster rate than would occur in a landing in which the pilot utilized manual brakes. This characteristic has been considered undesirable by some pilots since they feel that excessive elevator control is required to prevent a hard nose-gear touchdown. This problem has two roots:
(a) Brake application prior to nose-gear touchdown. Application of main-gear brakes will always impose a pitch-down moment on the aircraft. Present autobrake systems will apply brake pressure immediately after main-gear touchdown whereas a pilot will generally not apply manual brakes until after the nose gear is on the ground. Application of even low-to-moderate levels of braking will cause a rapid derotation of the aircraft if the nose gear is not yet on the ground and is unable to react the resultant pitch-down moment on the aircraft. The pitch-down moment prior to nose-gear touchdown can be reacted only by nose-up elevator control. PA1 (b) Deceleration error due to pitch attitude. This particular problem is further aggravated by use of an accelerometer for feedback control of deceleration by the autobrake system. The accelerometer mounted parallel to the aircraft's longitudinal axis is sensitive to pitch attitude such that the indicated deceleration includes not only the deceleration due to change in aircraft velocity but also includes the component of the gravity vector which is resolved onto the aircraft's longitudinal axis. For small nose-up pitch angles, this gravity component is proportional to pitch angle and is opposite to the direction of travel such that the indicated deceleration is less than the actual deceleration. The deceleration error due to pitch attitude may be equal in magnitude to the commanded deceleration but opposite in sign and will cause the autobrake system to substantially increase the brake pressure in order to meet the commanded deceleration. This worsens the derotation characteristics of the autobrake system. PA1 (a) Brake application prior to nose-gear touchdown. Prior autobrake systems have provided a solution to this problem by commanding a fixed low value of deceleration prior to nose-gear touchdown, then commanding the pilot-selected deceleration after nose-gear touchdown. PA1 (b) Deceleration error due to pitch attitude. Certain prior autobrake systems utilized wheel speed rather than accelerometers to determine deceleration. As a result, these systems were not subject to a pitch attitude error. Reverting to a wheel-speed-derived deceleration signal would be unsatisfactory since a wheel deceleration signal is not as accurate as an accelerometer signal. An attempt has been made to use open-loop compensation for pitch attitude by assuming a fixed pitch angle for the landing flare attitude followed by a fixed derotation schedule. This approach was unsatisfactory in that it was not suited for cases where the pilot brought the nose down either faster or slower than the assumed derotation rate. A prior autobrake system utilizes an accelerometer for autobrake control but does not correct the deceleration data for pitch attitude. PA1 (a) Determine nose-gear touchdown. Nose-gear touchdown is assumed when the pitch angle is less than one degree. Prior to nose-gear touchdown, a preferred embodiment of the present autobrake system will increase the aircraft deceleration to some fixed low value over a period of several seconds. The deceleration level is selected to minimize the amount of elevator control required to counter the pitch-down moment due to braking while still providing a small amount of braking. After nose-gear touchdown the commanded deceleration level becomes that which was selected by the flight crew. Once pitch angle has become less than one degree, even momenterily, the autobrake computer will continue to assume nose-gear touchdown regardless of any subsequent increase in pitch angle to a value greater than one degree. A time-out is utilized to switch the autobrake control to the level commanded by the flight crew in case the nose is held off, or the indicated pitch attitude exceeds one degree for more than eight seconds following main-gear touchdown. PA1 (b) Correct the indicated deceleration data for pitch attitude prior to nose-gear touchdown. This, in accordance with the present ihvention, eliminates the deceleration error arising from pitch attitude and prevents application of excessive braking due to this error, even though a low level of deceleration is commanded, during the derotation phase of the landing.
Prior attempts to solve the aforementioned problem resulted in:
These systems have utilized nose-gear squat switches to detect nose-gear touchdown. While the general approach of commanding a low deceleration level prior to nose-gear touchdown is satisfactory, use of a nose-gear squat switch to detect nose-gear touchdown is not possible on all present aircraft in that a nose-gear squat switch signal is not available to all the present autobrake systems. Additionally, nose-gear squat switches, when present, are installed in an exposed position and are prone to mechanical damage. The relatively high failure rate of the nose-gear squat switches, coupled with the difficulty of detecting the switch failure on a routine basis make it desirable to find an alternative to the nose-gear squat switch to determine nose-gear touchdown.
An attempt to simply delay braking until it could be assumed that the nose-gear touchdown had occurred has proven unsatisfactory in cases where the pilot brought the nose down rapidly in order to make a short landing roll due to the built-in delay in significant braking. This was also unsatisfactory in cases where the pilot held the nose up longer than the assumed touchdown time delay in that application of brakes at that time still caused a rapid derotation of the aircraft.
As a consequence, it is an object of the present invention to provide an autobrake system for receiving aircraft pitch angle data from one of the aircraft's Inertial Reference Units (IRU) and then utilizing the pitch data to:
It is a further object of this invention to utilize pitch data to replace the need for a nose-gear squat switch and to eliminate the error due to pitch attitude in the deceleration data provided from the IRU.
It is another object of this invention to provide means for accurately controlling and limiting the level of deceleration produced by an aircraft automatic braking system during that period immediately after landing touchdown on the main gear and prior to nose-gear touchdown since excessive application of brake pressure while the nose gear is off the ground will produce a large nose-down pitching moment on the aircraft which can result in a high derotation rate objectionable to flight crew and passengers.
The foregoing, and other objects and advantages of this invention, will best be understood by the following detailed description of a preferred embodiment thereof taken in view of the accompanying drawing wherein: